Newsgroups: rec.bicycles.tech,rec.bicycles.off-road
From: jbrandt@hpl.hp.com (Jobst Brandt)
Subject: Re: Shimano V-brake levers not Servo???
Date: Fri, 15 Dec 1995 22:01:02 GMT
John Olsen writes:
>> Because brake designers have spent years perfecting brakes that
>> have no servo effect, servo effect being the bane of all brakes.
>> The operator wants a constant response for a given application
>> force. Any servo effect perverts this control process and can
>> cause lockup. That is why cars went to disk brakes and heavy
>> trucks did not. Drums are better brakes but they inherently have a
>> servo effect. "My brakes locked up" used to be a common complaint
>> with drums. Trucks need the extra brake capacity and are not
>> driven as near the limit as passenger cars, so they continue to use
>> drums.
> Cars went to discs to improve heat dissipation, first and foremost.
> Trucks have stayed with drums primarily because they are traditional
> and in the system. Disc brakes have been available on trucks for 15
> years, and they work fine. With brake boosting, the non-linear
> servo effect of drums is irrelevant and hard to deal with. At this
> point, discs are more expensive, and much harder to find parts for in
> Buffalo Breath, Wyoming. Drums catch and hold contaminants like
> water and grunge much more than do discs, and they go away faster in
> off-road use.
I don't believe so. Disks were introduced to get rid of the self
energizing effect of drums. I worked as a design engineer on the
introduction of disks while working in the racing department of a
formula one team in the 1960's when disks were being developed. I am
aware of the design problems and the driver response. We were well
aware of the need for disks and it wasn't for heat dissipation. In
fact heat dissipation was a major drawback because no friction
materials were available that could operate at the temperatures disks
force on the design. The material we had did not work when cold and
the ones that worked cold didn't work above 1000 degrees F.
Disks on trucks present enormous heat dissipation problems because
there is insufficient area to cool them unless multiple disks are used
as on railways where as many as four 400mm diameter 100mm disks are
uses per axle of a passenger car. This is an enormous expense but is
necessary because skidding is so hard to control with any other brake.
There is nothing "traditional" about brakes on highly competitive long
haul trucking. Multiple disks are not a viable option for trucks.
> I think the main reason for non-linear cable pull levers is to park
> the pads farther from the rim (a low-leverage linkage feature) while
> still providing adequate leverage when the pad gets to the rim (a
> short-pad-travel feature). I hated the damn servowave levers,
> primarily because so many of them were and still are the horrible
> DiaCompe PC7 pieces of dung, which made cable swapping impossible.
> But, the point wasn't a great increase in at-the-rim leverage,but
> rather more cable pull with adequate leverage at the rim.
Two stage actuation is the holy grail of brakes. No one has yet been
able to make on whose transition point can track break wear. On cars,
this is taken care of by power brakes that achieve their free travel
by having a poor mechanical advantage throughout the stroke but use
power to make up for it. The limit is that when power fails, the
driver must still be able to stop. You ought to try that some time in
a safe place. If you turn off the engine and try to stop, you come
face to face with an extra ineffective brake.
Jobst Brandt <jbrandt@hpl.hp.com>

Newsgroups: rec.bicycles.racing,rec.bicycles.tech
From: jbrandt@hpl.hp.com (Jobst Brandt)
Subject: Re: Campag Delta Brakes
Date: Fri, 22 Mar 1996 21:12:17 GMT
Alan Bishop writes:
>> The reason disks are used on cars and motorcycles is singularly
>> because they decouple the servo effect of drums whose effect varied
>> so greatly over the operating...
> Jobst - you are one of the greatest destroyers of the unsupported
> assertion! But here you have abandoned all objectivity and rational
> thought and fallen guilty to the cheap shot.
Get to the point.
Either you are unclear on the meaning of "cheap shot" or I'm missing
what it is you are complaining about. Thanks for the evaluation, just
the same.
> If you do your sums, check out the physics and the mathematics of
> drum brakes and look at real usage, then you will find that the
> servo effect of a leading shoe drum brake is reliable, predictable
> and linear with speed. The magnitude of the servo effect varies
> with the coefficient of friction of shoe on drum and to some extent
> with shoe wear. If a drum brake gets wet (creek crossings etc) then
> the servo action falls away and brakes become ineffective. Same
> with brake fade.
I don't know why you want to believe that or what your background in
brake design is. I was a development engineer at Porsche in Germany
when Girling and ATE were introducing disks for cars. We were already
using them on the F1 car for which I was responsible for the
suspension brakes and steering. It may have gotten lost in the shuffle
after disks were introduced but the urgency of disks was linearity and
uniformity of response.
It was not considered unusual before the advent of disks that people's
drum brakes would, on occasion, lock up when lightly applied. I have
experienced this. It is for this unpredictable servo action that
railroads never pursued expanding shoe brakes. Today they use disks
at great expense.
> In all other respects, drum brakes were/are effectively linear,
> manageable and useful.
If you accept that there is a servo effect in drum brakes and that
the coefficient of friction is humidity and temperature sensitive,
then your statement must be incorrect. Those in engineering of brakes
will all tell you that this is the case and that servo is the bane of
brake control.
Jobst Brandt <jbrandt@hpl.hp.com>

Newsgroups: rec.bicycles.tech
From: jbrandt@hpl.hp.com (Jobst Brandt)
Subject: Re: Rear Disc Brake for T
Date: Mon, 20 May 1996 19:02:56 GMT
Scott Laughlin writes:
>> I guess you don't drive a car. Disks on cars get hot and cool off,
>> even in the rain and splashing through puddles. I don't know what
>> sort of warping would frighten you. Warp, if it occurred, might
>> cause some drag, but dangerous, it is not.
> Warping of brakes for overheating can set up a vibrations . A dial
> indicator will show you how much run out there is in a brake disk.
> There are manufacturers tolerances. They shouldn't be ignored.
Warped brakes cause drag, not vibrations. The vibrations in auto disk
brakes are almost exclusively metallic orientation in the casting of
the disk that becomes variably slick and rough, and if driven long
enough, can cause a variable thickness disk. The turning of disks is
primarily done to remove the variable surface and only coincidentally
trues the rotor. Brake calipers are all designed to align to warped
or shifted disks.
> I can't think of any friction material that was designed to operate
> at a temperature where brakes fade and components glow from heat.
> Anyone who allows this to happen repeatedly is certainly looking for
> some serious problems. It would be my choice to control my speed
> before things have reached that point.
It is not designed to fade, but it does. Drum brake friction material
must be soft enough to retard under a relatively low unit pressure
over the entire shoe. Disks having a far smaller brake pad can
operate at a higher pressure and can be harder, withstanding higher
temperature. I think this has been covered.
Jobst Brandt <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: dual pivot brakes better?
Date: 5 Jan 2000 18:35:51 GMT
Stephe (who?) writes:
>> From the FAQ:
>> --------------------------------------------------------------------
>> With self servo, the relationship between application force and
>> brake response is non linear and unpredictable. To escape this
>> self servo effect in drum brakes, automobiles and motorcycles
>> switched to disk brakes,
> LOL
> This is the funniest thing I've ever read on the net!!!! Where do
> these people find/makeup this stuff? They designed car brakes to
> HAVE servo action, that isn't something they tried to avoid. That's
> what the whole floating adjuster, leading-trailing shoe drum system
> was about.
What is it that you find funny about this? The servo action was an
undesirable necessity before the advent of power brakes and is why
disk brakes were introduced. Disks have the primary advantage of no
servo effect so that response is predictable and linear. Highway
trucks still use drums because they can get away with it. No railway
uses drums because the self servo effect will always cause skids,
skids that ruin a rail wheel with a flat spot.
> Datsun even designed servo disk brakes on the early 510s to save the
> cost of a brake booster.
There are always some people who make major technical errors for lack
of understanding the problem. I don't know that Datsun produced such
a brake but I'm sure it didn't go far. Brakes are generally developed
by a car manufacturer in conjunction with a brake company, a company
that understands the problem. This is not the case in the bicycle
business.
> While I can see it might not be a good idea on a bicycle, servo
> action had NOTHING to do with why they went to disk brakes on cars,
> it was solely an overheating problem with drums that warranted this
> change.
Oh? How do you know that? I gathered my understanding of brakes
while developing early disc brakes as a member of R&D at Porsche
where, in conjunction with ATE Dunlop, I worked on the design of discs
on the 804 F1 car. Rather than point out how dangerous drum brakes
were, emphasis was on something people could identify with, such as
how much more powerful disks were. Just try and tell the man on the
street that non linear servo effect is bad for braking. What you'll
get is a bunch of jive, like that which we see here on wreck.bike.
As I said, you'll still find drum brakes on highway trucks because
they are the most economical and volumetric effective brake with the
least maintenance, even though they do not respond as one would like.
Evidence to this problem are long dual skid marks common on roads that
are caused by the servo action of drum brakes. Older drivers will
recall the cause of various accidents in which the brakes locked up.
That is an extreme case that occurred often in the days of yore.
> The idea's and info some of the people here come up with is ALWAYS
> good for a laugh or two.
> Keep it up!
Don't stick your foot in your mouth so readily so you can laugh better.
Jobst Brandt <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: dual pivot brakes better?
Date: 5 Jan 2000 21:52:42 GMT
Jon Isaacs writes:
>> As I said, you'll still find drum brakes on highway trucks because
>> they are the most economical and volumetric effective brake with
>> the least maintenance, even though they do not respond as one would
>> like. Evidence to this problem are long dual skid marks common on
>> roads that are caused by the servo action of drum brakes. Older
>> drivers will recall the cause of various accidents in which the
>> brakes locked up. That is an extreme case that occurred often in
>> the days of yore.
> I believe that truck brakes (i could be wrong, the last brake job I
> did on an 18 wheeler was in 1983) are not servo brakes but rather
> use fixed pivots for both shoes. The rotary S-Cam or Wedge
> actuators in conjunction with the large 3 to 4 inch 120 psi air
> cylinders allow trucks to use non-servo brakes and thus avoid the
> fade problems associated with servo brakes.
All internally expanding drum brakes have a self servo effect
especially ones with leading edge hinged brake shoes (the way they are
commonly used) and even more so with floating shoes. A vector diagram
of the retarding force on a brake shoe shows that it transfers some of
that force in the direction of contact pressure. To do otherwise
makes the brake so ineffective that excess force on the order of that
used by disk brakes. You can see this in a diameter ratio comparison
of of drum and disk brake master and wheel cylinders. Drums use their
inherent servo effect to make far smaller wheel cylinders do the job.
I think the external diaphragm and lever drum brake is still the most
common brake on highway trucks. This brake uses conventional hinged
brake pads activated by a cam on a shaft rotated by the air actuator.
> However this does not solve the problem of keeping the brakes from
> overheating. Whereas a servo brake will begin to fade as they
> overheat, Truck brakes (experience as a driver with 80,000 plus
> pounds and no Jake brake) do not fade significantly but do not have
> sufficient surface area to remain cool when descending a long grade.
> They are subject to overheating and even can catch on fire.
The reason one doesn't use disks is that disks have less cooling area
than the large drums used on trucks. Railway HST cars have axles with
four 36" diameter 6" thick vented disks because they must contain all
the power of one emergency stop without heat rejection. The cooling
is mainly to cool the brake before the next application. Once
skidding, a railway wheel has almost no resistance as it rides on a
fluid film of molten steel. Therefore, ABS is a must for all HST.
Truck brakes cannot be used as a primary means of descending a long
grade unless speed is held to a crawl. Some form of dynamic braking
such as a water pump energy dissipater and large radiator or as you
mention a compression brake that produces hot air. Diesel electric
railway locomotives back drive their water cooled traction motors and
blow off the energy through large resistor banks and huge fans that
make the howling sound one hears when freight trains descend.
> As far as the skid marks of duals locking up that one sees on the
> highway, my experience has been that this is usually caused by
> equipment failure that went unnoticed by the driver. Either a
> ruptured air line or wheel bearing problem can cause this. Since
> truck brakes are dual systems, ie use air to release them (unlock
> from parking position) and air to actuate them, failure of the
> release line can cause lockup.
I think you'll find that, in stop and go traffic, trucks regularly
skid wheels because control is not sufficient to make a non-skid stop.
I see white smoke from truck tires often in heavy traffic and I'm sure
they are not all suffering some mechanical error.
> If a truck has to stop quickly, then the brakes will certainly lock
> causing long skid marks, especially if the driver dumps the release
> (parking valve) for some reason. The force which the actuators can
> apply if need be I think are sufficient to always lock the wheels.
This is not the best way to slow down because retardation is worse
than when not skidding because the tires melt in these situations.
That they melt is evident from the skid prints left by the tire when
it again rotates leaving a solid tread print on the road for several
rotations.
> Truck tires are notoriously hard, running at bicycle type pressures
> of over 100 psi and in order to have the tires last a long time,
> they use rubber designed for long life rather than traction. Thus
> skidding and long stops are the norm.
The rubber is not necessarily harder although the casings are
stronger. Harder rubber does not necessarily reduce wear, in fact, it
increases wear above a certain level because contact pressure is more
concentrated, increases scrubbing as the tire lifts off with less
longitudinal compliance.
> So, enough of my nostalgia and such. I think that the reason that
> trucks (20,000lb/axle trucks) still use drum brakes is that they are
> not servo brakes.
Well I think that is a matter of definition. Drum brakes are non
linear and are sensitive to small changes in friction coefficient.
Disks are superior, not because they can dissipate more power but
because their actuation force is entirely decoupled from the retarding
force. Passenger cars are not run at glowing temperatures although
they can reach that at the expense of considerable brake fade.
Jobst Brandt <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: dual pivot brakes better?
Date: 6 Jan 2000 01:14:46 GMT
Stephe (who?) writes:
>> All internally expanding drum brakes have a self servo effect
>> especially ones with leading edge hinged brake shoes (the way they
>> are commonly used) and even more so with floating shoes.
> So if this is such a bad thing why did they use these floating
> shoes on almost every American car at the end of drum brakes?
As I said, before the days of power brakes this was the only way to
stop a large car. Even after power brakes, stopping the car with a
dead engine was a consideration and for that event, leading floating
shoe brakes were necessary. Today we assume that power brakes and
engines are reliable enough to let people drive cars that cannot be
readily steered and braked without power assist.
> Oh I know **YOU** know more about this than all of them put
> together? (snicker)
> The MAIN problem with drum brakes is the overheating problem.
Not so. With the low application pressure and large brake shoe, a
soft material was required to give adequate braking. Soft material
has higher volatility and a lower melting point. With power brakes as
trucks have, they can be run far hotter than passenger cars of old. By
the way, where did you pick up your ideas on brakes. They are a bit
off the mark.
> Drum brakes work fine (if sized correctly) until they have to
> make multiple stops (road racing / heavy traffic). That's why
> they work fine on the rear of cars as they don't have too much
> heat to deal with.
That's not the reason. Making a hand brake with disks is difficult so
most cars use drums on the rear, where they don't do much anyway, in
order to have a drum hand brake. Expensive cars use an auxiliary hand
brake and use disks on all wheels.
> I've road raced old datsun Z cars with drum brakes on the rear and
> had NO problem with them locking or being non linear. The reason
> they waited so long to use disks was purely a cost savings for the
> manufacturer. No one is arguing that disk brakes aren't better, but
> to claim that the lack of servo action is why they changed is
> comical. I could really care less where you worked or what school
> you went to, this is BS...
I don't mind that but you should care about what you don't know about
brakes. Your barging in on this newsgroup with all the old curbstone
mechanic tales is like the guy who rides up to a group of to show them
how fast he is, only to discover that these are the fast riders taking
it easy. You might look around a bit and find out what's going on
before blasting off. We've heard your take on mechanical things here
before... often.
> BTW I'm not the one with my foot in my mouth..
You ought to check on that from time to time.
Jobst Brandt <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: dual pivot brakes better?
Date: 6 Jan 2000 20:03:30 GMT
Alex Rodriguez writes:
> I get the distinct impression that the term "servo" is being used
> differently by Jobst and Stephe and that may be part of the
> confusion. I've been trying to follow this discussion and I'm not
> sure I understand this term myself.
In this context, servo effect means that there is an interaction
between application and friction (braking) force, so that friction
force modifies the application force, thereby using feedback to
magnify the application force. In an internal shoe drum brake, where
a semicircular shoe is anchored on its downstream end, called a
leading shoe, friction force pushes the shoe into the drum. If it is
attached on the opposite end, and I don't know of any that do this,
drum rotation works to disengage the shoe from contact.
This is similar to pulling or pushing a sliding stick angled to a
road. Pulled, it lifts off and makes light contact, pushed its
contact force increases if it meets with sliding resistance, which in
turn increases the contact force, a process that readily locks up.
The effect can be light or severe, depending on the angle of contact.
The same stick pressed vertically onto the road in a rigid guide has
no servo effect and has a drag force directly related to its contact
force, a force that remains unmodified by the friction force.
In a drum brake, the contact angle (of the stick to road) is given by
how near the brake shoe pivot is to the center of the drum.
Jobst Brandt <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: dual pivot brakes better?
Date: 7 Jan 2000 23:27:33 GMT
Roger Cantwell writes:
>>> The servo action is designed to provide a self-power because the
>>> shoes force themselves into the drums as the drums turn.
>> One infrequently-experienced problem this brings is that one gets
>> very poor braking in reverse. I recall rallying in a mark 1 Ford
>> Escort (the European type) many years ago. We overshot a turn on a
>> tarmac road. Nothing daunted, I whapped in the best handbrake turn
>> of my life, leaving us going backwards down the road at about
>> 40mph. At which point I discovered that the (all drum) brakes
>> basically had little or no effect...
> Must have been a 2LS brake - these hardly work in reverse at all
> precisely because there's no self-servo. They should be used on the
> front only.
Not exactly. The servo is there but it is negative. That is why it
is so bad. If there were no feedback the brake would be neutral to
direction.
Jobst Brandt <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: dual pivot brakes better?
Date: 7 Jan 2000 18:11:55 GMT
Peter Headland writes:
>> The servo action is designed to provide a self-power because the
>> shoes force themselves into the drums as the drums turn.
> One infrequently-experienced problem this brings is that one gets
> very poor braking in reverse. I recall rallying in a mark 1 Ford
> Escort (the European type) many years ago. We overshot a turn on a
> tarmac road. Nothing daunted, I whapped in the best handbrake turn
> of my life, leaving us going backwards down the road at about 40mph.
> At which point I discovered that the (all drum) brakes basically had
> little or no effect...
I once had that experience while doing a NASCAR type bailout when my
rear engine car got crossed up. In such a case, locking all four
wheels causes the car to slide like a brick down the road to a safe
halt regardless of which way the car is pointing. Being aware of this
as the car spun around, I pressed on the brake pedal extra hard as the
slide went "backward" as I felt the wheels start to turn and try to
put an unwanted direction into the trajectory. This maneuver is used
on tracks and can be seen regularly by the immediate cloud of white
smoke from a car that loses control in hard-top racing. With all
wheels stopped, the car takes a predictable and non intrusive
trajectory that other cars can cope with better than one that is
rolling across the track after getting crossed up.
Jobst Brandt <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: dual pivot brakes better?
Date: 7 Jan 2000 16:16:08 GMT
evans (who?) writes:
>> In an internal shoe drum brake, where a semicircular shoe is
>> anchored on its downstream end, called a leading shoe, friction
>> force pushes the shoe into the drum. If it is attached on the
>> opposite end, and I don't know of any that do this, drum rotation
>> works to disengage the shoe from contact.
> Almost every motorcycle rear drum brake I've seen has one leading
> and one trailing shoe (exception was the Honda CB77).
But not with only trailing shoes. The combination of leading trailing
is an inexpensive single fixed pivot design that is good enough for a
rear brake because it doesn't do much anyway. Floating the shoes is
more complex and double leading shoes requires two hydraulic cylinders
or cams. I don't think you'll find trailing shoes on the front wheels
of any current high performance vehicle. That's something for golf
carts.
Jobst Brandt <jbrandt@hpl.hp.com>